ISO 20806:2009

INTERNATIONAL ISO
STANDARD 20806
Second edition
2009-09-15

Mechanical vibration — Criteria and
safeguards for the in-situ balancing of
medium and large rotors
Vibrations mécaniques — Critères et sauvegardes relatifs à
l'équilibrage in situ des rotors moyens et grands




Reference number
ISO 20806:2009(E)
©
ISO 2009

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ISO 20806:2009(E)
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ISO 20806:2009(E)
Contents Page
Foreword .iv
Introduction.v
1 Scope.1
2 Normative references.1
3 Terms and definitions .2
4 In-situ balancing .2
4.1 General .2
4.2 Reasons for in-situ balancing .2
4.3 Objectives of in-situ balancing .3
5 Criteria for performing in-situ balancing .3
6 Safeguards .4
6.1 General .4
6.2 Safety of personnel while operating close to a rotating shaft.4
6.3 Special operating envelope for in-situ balancing .4
6.4 Integrity and design of the correction masses and their attachments.4
6.5 Machinery-specific safety implications.5
7 Measurements .5
7.1 Vibration measurement equipment .5
7.2 Measurement errors .6
7.3 Phase reference signals .6
8 Operational conditions .8
9 Reporting.8
9.1 General .8
9.2 Report introduction .8
9.3 Vibration measurement equipment .9
9.4 Results.9
9.5 Text information .11
Annex A (normative) Precautions and safeguards for specific machine types during in-situ
balancing.12
Annex B (informative) Example of an in-situ balancing report for a boiler fan < 1 MW.13
Annex C (informative) Example of an in-situ balancing report for a large > 50 MW turbine
generator set.17
Bibliography.23

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ISO 20806:2009(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies
(ISO member bodies). The work of preparing International Standards is normally carried out through ISO
technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 20806 was prepared by Technical Committee ISO/TC 108, Mechanical vibration, shock and condition
monitoring.
This second edition cancels and replaces the first edition (ISO 20806:2004), of which it constitutes a minor
revision.
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ISO 20806:2009(E)
Introduction
Balancing is the process by which the mass distribution of a rotor is checked and, if necessary, adjusted to
ensure that the residual unbalance or the vibrations of the journals/bearing supports and/or forces at the
bearings are within specified limits. Many rotors are balanced in specially designed balancing facilities prior to
installation into their bearings on site. However, if remedial work is carried out locally or a balancing machine
is not available, it is becoming increasingly common to balance the rotor in situ.
Unlike balancing in a specially designed balancing machine, in-situ balancing has the advantage that the rotor
is installed in its working environment. Therefore there is no compromise with regard to the dynamic
properties of its bearings and support structure, nor from the influence of other elements in the complete rotor
train. However, it has the large disadvantage of restricted access and the need to operate the whole machine.
Restricted access can limit the planes at which correction masses can be added, and using the whole
machine has commercial penalties of both downtime and running costs. Where gross unbalance exists, it may
not be possible to balance a rotor in situ due to limited access to balance planes and the size of correction
masses available.
A general guide to the International Standards associated with mechanical balancing of rotors is given in
[4]
ISO 19499 . Rotors with a constant (rigid) behaviour are covered by ISO 1940-1 and rotors with a shaft
[3]
elastic (flexible) behaviour are covered by ISO 11342 .

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INTERNATIONAL STANDARD ISO 20806:2009(E)

Mechanical vibration — Criteria and safeguards for the in-situ
balancing of medium and large rotors
1 Scope
This International Standard specifies procedures to be adopted when balancing medium and large rotors
installed in their own bearings on site. It addresses the conditions under which it is appropriate to undertake
in-situ balancing, the instrumentation required, the safety implications and the requirements for reporting and
maintaining records.
This International Standard can be used as a basis for a contract to undertake in-situ balancing.
It does not provide guidance on the methods used to calculate the correction masses from measured vibration
data.
NOTE The procedures covered in this International Standard are suitable for medium and large machines. However,
many of the principles are equally applicable to machines of a smaller size, where it is necessary to maintain good records
of the vibration behaviour and the correction mass configurations.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including amendments) applies.
ISO 1925, Mechanical vibration — Balancing — Vocabulary
ISO 1940-1, Mechanical vibration — Balance quality requirements for rotors in a constant (rigid) state —
Part 1: Specification and verification of balance tolerances
ISO 2041, Mechanical vibration, shock and condition monitoring — Vocabulary
ISO 2954, Mechanical vibration of rotating and reciprocating machinery — Requirements for instruments for
measuring vibration severity
ISO 7919 (all parts), Mechanical vibration — Evaluation of machine vibration by measurements on rotating
shafts
ISO 10816 (all parts), Mechanical vibration — Evaluation of machine vibration by measurements on non-
rotating parts
IS0 10817-1, Rotating shaft vibration measuring systems — Part 1: Relative and absolute sensing of radial
vibration
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ISO 20806:2009(E)
3 Terms and definitions
For the purpose of this document, the terms and definitions given in ISO 1925 and ISO 2041 apply.
3.1
in-situ balancing
process of balancing a rotor in its own bearings and support structure, rather than in a balancing machine
NOTE Adapted from the definition of “field balancing” in ISO 1925:2001, 4.14. As it is easier to understand, the term
“in-situ balancing” is to replace “field balancing” in the next revision of ISO 1925.
4 In-situ balancing
4.1 General
For in-situ balancing, correction masses are added to the rotor at a limited number of conveniently engineered
and accessible locations along the rotor. By doing this the magnitude of shaft and/or pedestal vibrations
and/or unbalance is reduced to within acceptable values, so that the machine can operate safely throughout
its whole operating envelope. As part of a successful balance, transient speed vibration may be compromised
to some degree to obtain acceptable normal running speed vibration on a fixed speed machinery train.
NOTE In certain cases, machines that are very sensitive to unbalance cannot be successfully balanced over the
complete operating envelope. This usually occurs when a machine is operating at a speed close to a lightly damped
[2]
system mode (see ISO 10814 ) and has load-dependent unbalance.
Most sites have limited instrumentation and data-processing capabilities, when compared to a balancing
facility, and additional instrumentation is required to undertake in-situ balancing in these situations. In addition,
the potential safety implications of running a rotor with correction masses shall be taken into account.
4.2 Reasons for in-situ balancing
4.2.1 Although individual rotors may be correctly balanced, as appropriate, in a high- or low-speed
balancing machine, in-situ balancing might be required when the rotors are coupled into the complete rotor
train. This could be due to a range of differences between the real machine and the isolated environment in
the balancing machine, including:
a) a difference in dynamic characteristics of the rotor supports between the balancing facility and the
installed machine;
b) assembly errors that occur during installation, which cannot be reasonably found and corrected;
c) rotor systems that cannot be balanced prior to assembly;
d) a changing unbalance behaviour of the rotor under full functional operating conditions.
4.2.2 Balancing might also be required to compensate for in-service changes to the rotor, including:
a) wear;
b) loss of components, such as rotor blade erosion shields;
c) repair work, where components could be changed or replaced;
d) movement of components on the rotor train causing unbalance, such as couplings, gas turbine discs and
generator end rings.
NOTE Rotor blades are normally added as balanced sets, but this may not be possible if a small number of blades
are replaced.
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ISO 20806:2009(E)
4.2.3 In-situ balancing might be necessary due to a range of economic and technical reasons, including:
a) the investment in a balancing machine cannot be justified;
b) when a suitable balancing machine is not available in the correct location or at the required time;
c) when it is not economic to dismantle the machine and transport the rotor(s) to a suitable balancing facility.
4.2.4 Machines under normal operation and/or during speed variations (following remedial work, or after
commissioning) might have unacceptable magnitudes of vibration when compared with common practice,
contractual requirements, or International Standards such as ISO 10816 (all parts) and ISO 7919 (all parts). In
many cases, it may be possible to bring the machine within acceptable vibration magnitude by in-situ
balancing.
4.3 Objectives of in-situ balancing
The reason for balancing is to reduce the vibration magnitudes to acceptable values for long-term operation.
For most machines, the overall vibration magnitude limits shall either be based on common practice or the
appropriate part of ISO 10816 and ISO 7919 for pedestals and shafts, respectively.
Where the magnitude of unbalance is of concern, reduce the magnitude of unbalance to within permissible
limits (see ISO 1940-1 for details).
5 Criteria for performing in-situ balancing
Prior to in-situ balancing, a feasibility study shall be carried out to assess if the available correction planes are
suitable to influence the vibration behaviour being observed, since limited access to correction planes and
measurement points on the fully built-up machine can make in-situ balancing impractical. Where possible,
experience from previous in-situ balancing should be used. Sometimes modal analysis may be required.
In-situ balancing shall only be attempted in the following circumstances:
a) the reasons for the high vibrations are understood and cannot be corrected at the source;
b) after analysis of the vibration behaviour, it is judged that balancing is a safe and practical approach;
c) under the required normal operating conditions, the vibration vector is steady and repeatable prior to and
during in-situ balancing;
d) the addition of correction masses only affects the once-per-revolution component of vibration and,
therefore, in-situ balancing shall only be carried out if this is a significant component of the overall
vibration magnitude.
In special circumstances, where the once-per-revolution vibration component changes during normal
operation of the machine (such as thermally induced bends in generator rotors), it is possible to reach
acceptable balancing results across the operating envelope by adding correction masses. Here, with the
vibration magnitude at full speed, no load might be compromised to obtain an acceptable vibration magnitude
at full load. Again, this shall only be attempted if the reasons for the unbalance are understood.
NOTE When systems are operating in a non-linear mode, correction masses can affect other vibration components,
including both sub and high shaft speed harmonics.
The once-per-revolution component of vibration might not originate from unbalance but be generated from
system forces such as those found in hydraulic pumps and electric motors. Many defects, such as shaft
alignment errors and tilting bearings, can also contribute to the once-per-revolution component of vibration.
Such effects should not normally be corrected by balancing, since balancing is effective at only a single speed
and could mask a real system fault.
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ISO 20806:2009(E)
The first shaft order vectors of synchronous vibration should be sufficiently steady such that the amplitude of
the variation is not significant relative to the amplitude of the mean vibration vector.
Where sufficient design data of the rotor system are available, rotor dynamic modelling can be used to aid the
choice of suitable balance planes and correction mass combinations.
6 Safeguards
6.1 General
WARNING — In-situ balancing shall only be undertaken by a skilled team, including both customer
and supplier, who understand the consequences of adding trial and correction masses and have
experience of operating the machine. Failure to do this can place the whole machine and staff at risk.
6.2 Safety of personnel while operating close to a rotating shaft
While undertaking in-situ balancing, the machine is operated under special conditions, allowing access to
rotating components to add trial and final correction masses. Strict safety procedures shall be in place to
ensure that the machine cannot be rotated while personnel have access to the shaft and that no temporary
equipment can become entwined when the shaft is rotated.
6.3 Special operating envelope for in-situ balancing
Machines may be quickly run up and run down many times and can have unusual loading conditions during
the in-situ balancing exercise, which could be outside the normal operating envelope of a machine. Examples
for specific machine types that shall be taken into account are given in Annex A. It shall be established that
such operations are not detrimental to the integrity or the life of the whole machine.
However, as no general list of machine types can cover all situations, it is necessary to review individually the
integrity requirements for each in-situ balance.
6.4 Integrity and design of the correction masses and their attachments
When trial and correction masses are added, it shall be confirmed that they are securely attached and their
mountings are capable of carrying the required loads. The correction masses shall not interfere with normal
operation, such as coming into contact with stationary components due to shaft expansion. The correction
masses should be fitted in accordance with the manufacturer's instructions, if available.
Correction masses are often attached with bolts or by welding. It shall be ensured that neither the bolt holes
nor the welding process compromise the integrity of the rotor component to which the correction masses are
attached, or the function of the component, such as cooling. Furthermore, correction masses shall be
compatible with their operating environment, such as temperature and chemical composition of the
atmosphere.
Where possible, the total mass of the correction masses on each plane shall be minimized by consolidating
those added from previous balancing exercises. However, correction masses that have been added for
specific reasons (such as to balance the individual disc or counteract for blade root eccentricity errors) should
not be changed.
When correction masses are added to non-integral rotating components, these parts should be match marked
so that the proper assembly orientation can be maintained.
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ISO 20806:2009(E)
6.5 Machinery-specific safety implications
General safety requirements associated with in-situ balancing are discussed in 6.2 to 6.4, but precautions and
safeguards for specific machine types, given in Annex A, shall be taken into account. However, as no general
list of safety precautions can cover all machinery and all situations, it is necessary to review individually the
safety requirements for each in-situ balance.
7 Measurements
7.1 Vibration measurement equipment
Basic procedures for the evaluation of vibration by means of measurements made directly on the rotating
shaft shall conform to ISO 7919-1 and the measurement system shall conform to ISO 10817-1. Measurement
procedures for transducers mounted on the pedestal shall conform to ISO 10816-1 and the measurement
system shall conform to ISO 2954. Either system shall have sufficient frequency range to capture data for the
full speed range over which the machine is to be balanced. The transducers shall have the necessary
sensitivity and shall be located at the appropriate positions to measure the effects of the correction masses.
On flexible support structures, pedestal measurements may give the best results. On rigid supports, shaft
relative transducers can be more responsive. Guidance as to the most suitable measurement system can also
be gained from previous experience and rotor dynamic modelling. When eddy current non contact transducers
are used to measure the shaft relative motion the signal might be compromised by electrical and/or
mechanical run out of the measurement track (for details, see ISO 7919-1 and ISO 10817-1). Where these
effects significantly influence the true reading the source should be isolated and appropriate corrections made.
If available, shaft absolute measurement can be used, which provides a shaft position independent of the
pedestal movement.
ISO 7919 (all parts) and ISO 10816 (all parts) are concerned with acceptable overall vibration values for
machinery operating under steady-state conditions. For balancing, the vibration measurement equipment shall
have the additional facility to extract the once-per-revolution component of vibration, giving both amplitude and
phase. Furthermore, ISO 7919 (all parts) and ISO 10816 (all parts) apply to the radial measurement directions
on all bearings and the axial direction for only the thrust bearing. However, in some special conditions, axial
measurements on other bearings shall be carried out where necessary.
In-situ balancing is normally carried out to reduce the vibration magnitude at the operating speed and while
passing through the system resonances, during run up and run down. The measurement equipment shall
have sufficient dynamic range to measure both amplitude and phase over the full speed and operating ranges
under consideration.
Vibration shall be measured at selected locations where it is necessary to reduce its magnitude. However,
balancing can improve the vibration magnitude at some locations or directions at the expense of others.
Therefore, it is recommended to have additional transducers on adjacent rotors or bearings. Whilst, for
monitoring purposes, measurements in only one direction may be sufficient, for an in-situ balance it is
advisable to measure in two orthogonal directions, where possible.
Where permanently installed transducers are used, it is advisable to check their calibration, in both amplitude
and phase, immediately prior to balancing. Permanently installed shaft relative transducers are not normally
checked for calibration, but a phase and shaft run out check is advisable. It is normally sufficient to check the
phase of the shaft transducers by ensuring the signal has the correct polarity. Where accessible, pedestal
transducers shall be checked against portable equipment.
NOTE In some cases, it can be useful to measure the full orbit of vibration and in this instance it is necessary to have
pairs of transducers at selected axial measurement locations along the shaft. Strictly, it is only necessary to have two non-
parallel transducers to describe the orbit; however, orthogonal pairs are usually used.
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ISO 20806:2009(E)
7.2 Measurement errors
Any measurement is subject to error, which is the difference between the measured value and the true value.
The difference is called the error of measurement and, in balancing, this is caused by a combination of
systematic, randomly variable and scalar errors. Systematic errors are those when both amplitude and phase
of the unbalance can be evaluated by either calculation or measurement. Random errors are those when both
the amplitude and phase of the unbalance can vary unpredictably, and scalar errors occur when the
magnitude of the unbalance can be evaluated or estimated but the angle is undefined.
[1]
ISO 1940-2 gives examples of typical errors that can occur in the field of balancing and provides procedures
for their determination. Some of the examples presented are for the balancing facility, but many are also
applicable to in-situ balancing.
The limit for these errors shall be matched to the acceptance criteria of the in-situ balancing, as agreed
between the supplier and customer (see 4.3).
7.3 Phase reference signals
7.3.1 General
A phase reference mark, such as a keyway or reflective tape, is usually placed on the shaft or any
synchronous part, and is detected by a transducer mounted on a non-rotating component, such as a bearing
pedestal. This provides a once-per-revolution signal from which the phase of the vibration can be measured.
Sometimes the reference mark is permanently installed. The reference mark, such as a keyway or markings
on the shaft, shall be clearly documented and, if possible, shall be visible to allow correction masses to be
accurately placed.
In addition, the direction of shaft rotation shall be established so that phase angles, with or against rotation,
can be translated into the appropriate correction mass locations. Measured angles with rotation (phase lead)
require correction masses to be located in the direction of rotation from the leading edge of the phase mark.
Angles measured against rotation (phase lag) require the correction mass to be located against the direction
of rotation from the leading edge of the phase mark.
Alternative phase definitions may be adopted, but the system used shall be clearly defined. It is good practice
to ensure that the phase angle used for the location of the correction mass is consistent with the phase angle
of the once-per-revolution vibration.
7.3.2 Information required for reproducible phase reference data
The position of the shaft phase reference shall be consistently defined to provide accurate records so that
previous and future in-situ balancing data can be compared (see Clause 9). The pulse generated by the shaft
mark shall be sharp so that different trigger levels do not lead to inaccurate phase measurements. The
sinusoidal type signal (see Figure 1) can give a trigger time dependent on the level of the trigger setting, but
the sharp pulse (see Figure 2) gives a trigger time independent of the trigger voltage setting. Triggering shall
be from the leading edge of the pulse, for either negative or positive going pulses (either negative or positive
slope). Triggering on the trailing edge could lead to significant phase errors, since
...